1. Scanning Transmission
Electron Microscope

2. Outline
• History
• Interactions of Electrons
• Background
• STEM
• Components
• Specimen Preparation
• Mode
• Image formation
• Comparison

4. Cont…
• The first STEM was built in 1938 by Baron
Manfred von Ardenne.(distroyed in air raid)
• Not developed further until the 1970.
• Then Albert Crewe at the University of
Chicago developed the field emission gun.
• Then built a STEM able to visualize single
heavy atoms on thin carbon substrates

5. Interactions of electrons

7. Back ground
• Maximum resolution, d
• STEM an electron’s velocity approaches
the speed of light, c

8. STEM
• The basic principle of
image formation
fundamentally different
from static beam TEM
• small spot size is formed
on the sample surface
with the condenser
lenses
• This probe is scanned on
the sample surface
• the signal is detected by
an electron detector,
amplified and

9. Cont…
• DETECTOR
1. Small disk on the column
axis which detects the
transmitted beam (BF STEM
image) or diffracted beam
(DF STEM image)
2. Annular detector (a plate
with a hole) which detects all
the diffracted beams except
the transmitted one (ADF
STEM)
• Resolution
 limited by the spot size
have poorer resolution but

11. Scattered beam electrons
• In STEM signal is detected by
I. back scattered electrons(BSE)
II. Transmitted beam electrons scattered at
some angles
• In both cases, BSE and TBE, the signal
intensity is a function of the average
atomic number of the sample volume and
also phase contrast that interacted with
the beam
• Thus providing atomic number and phase

12. Cont…
• In STEM, the
electron beam is
rastered (scan coil)
across the surface of
a sample in a similar
manner to SEM,
however, the sample
is a thin TEM section
and the diffraction
contrast image is
collected on a solid-
state (ADF) detector.
HAADF-high angle
annular dark-field

15. Components
• Source formation
- Field emission
• Electromagnetic Lenses
- Condenser lens
• Aperture
• Specimen stage
-1. Single-tilt 2. Double-tilt
• Vacuum system
• Scanning coils
• Detectors
-1. BF 2.ADF3.HAADF

16. Source formation
• The STEM consists of an emission source
tungsten filament, or a lanthanum
hexaboride
• High voltage source (typically 100-300kV)
• Electrons emit by field emission.

17. Vacuum system
• STEM is evacuated to low pressure 10^ -4
Pa
• It consists of multiple pumping systems
and air locks.
• Low or roughing vacuum is achieved with
either rotary vacuum pump or diaphram
pumps
• For low vacuum turbomolecular pumps are
connected to the chamber
• Gate valve: for different vacuum levels in

18. V1 V2
Field
emission
Electromagnetic
Lenses
Aperture
Specimen stage
mesh

19. Specimen
holder

21. Specimen Preparation
• Preparation done in two steps
• Pre-Thinning:
 Reducing the thickness to about 0.1mm
• Final Thinning:
 Reducing the thickness to about 100nm
involve
Ion Milling
Electrolytic Thinning
Ultramicrotomy

22. Ion Milling
• Uses a beam of energetic
ions to bombard
specimen surfaces to
reduce the thickness by
knocking atoms out of a
specimen
• General procedure
a) Dimple grinding
b) ion milling
 ion beam of 1–10 keV
bombarded
 specimen is placed in
the center at an angle of

23. Electrolytic Thinning
• Reducing specimen
thickness to 100nm
• General procedure
A specimen placed in
an electrochemical
cell as anode
A suitable reduce
specimen thickness
Common technique is
jet polishing
Electrolytic thinning
completed in 3–15
minutes.

24. Ione
milling
Electrolytic
thinning
Ultramicrot
omy

26. Modes
• Transmitted electrons
that leave the sample at
relatively low angles
with respect to the optic
axis(bright field (BF).)
• Transmitted electrons
that leave the sample at
relatively high angles
with respect to the optic
axis(annular dark field
(ADF).)
• High Angle ADF
(HAADF) collects the

27. Image formation
• BF-STEM images
are equivalent to
TEM (reciprocity
principle).
• Produced Bragg
disks hitting the
detector
• Give the bright field
or phase signal

28. Image formation
Bright field STEM image of Au particles on a carbon film

29. ADF images
• Electrons which have
scattered to high
angles are collected
• Images contain
Bragg diffraction

30. HAADF images
• Two (out of several more) ways to
simulate HAADF-STEM images are
• Incoherent Imaging Model:
The Image is the convolution of object
potential and probe intensity.
Iimage (r )= Iprob (r ) V2proj (r )
• Multiple Scattering Image Simulation:
the frozen phonon approximation.

31. HAADF better Z-contrast than
BF
• HAADF is much less
sensitive to local
diffraction conditions
than BF.
• Its sensitivity mainly
to the atomic number

32. Bright and dark field STEM image of Au particles on a carbon film

33. Why use STEM?
•For DF imaging the annular detector collects more electrons than an
aperture.
•STEM ADF images are less noisy then TEM DF images as no lenses are
used to form them.
•Contrast in STEM images is greater than standard DF images.a) b) c)
• Comparison of TEM DF and STEM ADF images of the same sample
shows clear contrast difference